In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, dual connectivity communication has been specified. The dual connectivity communication is a communication mode in which a master cell group (MCG) and a secondary cell group (SCG) are set to a user terminal in an RRC (Radio Resource Control) connected mode. The MCG is a serving cell group managed by a maser base station. The SCG is a serving cell group managed by a secondary base station.
Further, in a scheme of transferring user data in the dual connectivity communication, a total of three types of bearers, that is, an MCG bearer, an SCG bearer, and a split bearer, are defined. The MCG bearer is a bearer in which a corresponding radio protocol is present only in the maser base station and which uses only a resource of the maser base station. The SCG bearer is a bearer in which a corresponding radio protocol is present only in the secondary base station and which uses only a resource of the secondary base station. The split bearer is a bearer in which a corresponding radio protocol is present both in the maser base station and the secondary base station and which uses both resources of the maser base station and the secondary base station.